This invention relates to methods for preparing calanolides and calanolide analogues.
Viruses, an important etiologic agent in infectious disease in humans and other mammals, are a diverse group of infectious agents that differ greatly in size, shape, chemical composition, host range, and effects on hosts. After several decades of study, only a limited number of antiviral agents are available for the treatment and/or prevention of diseases caused by viruses such as HIV, hepatitis B, herpes simplex type 1 and 2, cytomegalovirus, varicella zoster virus, Epstein Barr virus, influenza A and B, parainfluenza, adenovirus, measles, and respiratory syncytial virus. Because of their toxic effects on a host, many antiviral agents are limited to topical applications. Accordingly, there is a need for safe and effective antiviral agents with a wide-spectrum of anti-viral activity with reduced toxicity to the host.
Human immunodeficiency virus (HIV), which was also called human T-lymphotropic virus type III (HTLV-III), lymphadenopathy-associated virus (LAV) or AIDS-associated retrovirus (ARV), was first isolated in 1982 and has been identified as the etiologic agent of the acquired immunodeficiency syndrome (AIDS) and related diseases. Since then, chemotherapy of AIDS has been one of the most challenging scientific endeavors. So far, fourteen drugs have been approved by FDA and are being clinically used as drugs for the treatment of AIDS and AIDS-related complex. Although these FDA-approved drugs can extend the life of AIDS patients and improve their quality of life, none of these drugs are capable of curing the disease. Side effects as well as the emergence of drug-resistant viral strains limit the long-term use of these agents.1 On the other hand, the number of AIDS patients worldwide has increased dramatically within the past decade and estimates of the reported cases in the very near future also continue to rise dramatically. It is therefore apparent that there is a great need for other promising drugs having improved selectivity and activity to combat ADDS.1 Several approaches including chemical synthesis, natural products screening, and biotechnology have been utilized to identify compounds targeting different stages of HIV replication for therapeutic intervention.2 
The natural product screening program at the National Cancer Institute has discovered a class of remarkably effective anti-HIV compounds, named calanolides, from the rain forest tree Calophyllum lanigerum, with (+)-calanolide A, 1, being the most potent compound in the reported series.3 For example, (+)-calanolide A demonstrated 100% protection against the cytopathic effects of HIV-1, one of two distinct types of HIV, down to a concentration of 0.1 xcexcM. This agent also halted HIV-1 replication in human T-lymphoblastic cells (CEM-SS)(EC50=0.1 xcexcM/IC50=20 FM).3 More interestingly and importantly, (+)-calanolide A was found to be active against both the AZT-resistant G-9106 strain of HIV as well as the pyridinone-resistant A17 virus.3 Thus, the calanolides, classified 
as HIV-1 specific reverse transcriptase inhibitors, represent novel anti-HIV chemotherapeutic agents for drug development.
The hepatitis B virus (HBV) infects people of all ages. It is one of the fastest-spreading sexually transmitted diseases, and also can be transmitted by sharing needles or by behavior in which a person""s mucus membranes are exposed to an infected person""s blood, semen, vaginal secretions, or saliva. While the initial sickness is rarely fatal, ten percent of the people who contract hepatitis are infected for life and run a high risk of developing serious, long-term liver diseases, such as cirrhosis of the liver and liver cancer, which can cause serious complications or death.4. The World Health Organization lists HBV as the ninth leading cause of death. It is estimated that about 300 million persons are chronically infected with HBV worldwide, with over 1 million of those in the United States. The Center for Disease Control and Prevention estimates that over 300,000 new cases of acute HBV infection occurs in the United States each year, resulting in 4,000 deaths due to cirrhosis and 1,000 due to hepatocellular carcinoma.5 The highest rates of HBV infections occur in Southeast Asia, South Pacific Islands, Sub-Saharan Africa, Alaska, Amazon, Bahai, Haiti, and the Dominican Republic, where approximately 20% of the population is chronically infected.6 
Hepatitis B virus (HBV) infection is currently the most important chronic virus infection, but no safe and effective therapy is available at present. The major therapeutic option for carriers of HBV is alpha interferon, which can control active virus replication. However, even in the most successful studies, the response rate in carefully selected patient groups has rarely exceeded 40%.7,8 One of the reasons cited for interferon failure is the persistence of viral supercoiled DNA in the liver.9 Clinical exploration of many promising antiviral agents such as nucleoside analogues is hampered because their aspecific body distribution leads to significant toxic side effects. Recently, a new nucleoside analogue, 2xe2x80x2,3xe2x80x2-dideoxy-3xe2x80x2-thiacytidine (3TC), was approved to treat HBV infection with only minimal side effects. 10-12 
Influenza is a viral infection marked by fever, chills, and a generalized feeling of weakness and pain in the muscle, together with varying signs of soreness in the respiratory tract, head, and abdomen. Influenza is caused by several types of myxoviruses, categorized as groups A, B, and C4. These influenza viruses generally lead to similar symptoms but are completely unrelated antigenically, so that infection with one type confers no immunity against the other. Influenza tends to occur in wavelike epidemics throughout the world; influenza A tends to appear in cycles of two to three years and influenza B in cycles of four to five years. Influenza is one of the few common infectious diseases that are poorly controlled by modem medicine. Its annual epidemics are occasionally punctuated by devastating pandemics. For example, the influenza pandemic of 1918, which killed over 20 million people and affected perhaps 100 times that number, was the most lethal plague ever recorded. Since that time, there have been two other pandemics of lesser severity, the so-called Asian flu of 1957 and the Hong Kong flu of 1968. All of these pandemics were characterized by the appearance of a new strain of influenza virus to which the human population had little resistance and against which previously existing influenza virus vaccines were ineffective. Moreover, between pandemics, influenza virus undergoes a gradual antigenic variation that degrades the level of immunological resistance against renewed infection.13 
Anti-influenza vaccines, containing killed strains of types A and B virus currently in circulation, are available, but have only a 60 to 70% success rate in preventing infection. The standard influenza vaccine has to be redesigned each year to counter new variants of the virus. In addition, any immunity provided is short-lived. The only drugs currently effective in the prevention and treatment of influenza are amantadine hydrochloride and rimantadine hydrochloride.14-16 While the clinical use of amantadine has been limited by the excess rate of CNS side effects, rimantadine is more active against influenza A both in animals and human beings, with fewer side effects.17,18 It is the drug of choice for the chemoprophylaxis of influenza A.13, 19, 20 However, the clinical usefulness of both drugs is limited by their effectiveness against only influenza A viruses, by the uncertain therapeutic efficacy in severe influenza, and by the recent findings of recovery of drug-resistant strains in some treated patients.21-25 Ribavirin has been reported to be therapeutically active, but it remains in the investigational stage of development.26,27 
Cytomegalovirus (CMV) is a member of the herpes virus family, other well-known members of which include herpes simplex virus, types I and II, Epstein Barr virus and Varicella Zoster virus. Although these viruses are related taxonomically, all comprising double-stranded DNA viruses, infections due to these viruses manifest in clinically distinct ways. In the case of CMV, medical conditions arising from congenital infection include jaundice, respiratory distress and convulsive seizures that may result in mental retardation, neurologic disability or death. Infection in adults is frequently asymptomatic, but may manifest as mononucleosis, hepatitis, pneumonitis or retinitis, particularly in immunocompromised patients such as AIDS sufferers, chemotherapy patients and organ transplant patients undergoing tissue rejection therapy.
Up to 45% of all HIV-infected persons will develop cytomegalovirus-induced disease before their lives end.28 Although two antiviral agentsxe2x80x94ganciclovir and foscametxe2x80x94are available to treat human cytomegalovirus (HCMV), they act as virustatic agents to slow but not halt progression of disease; hence, disease routinely progresses despite daily maintenance with either agent. Moreover, therapy using either agent is problematic because both agents are associated with serious toxicities.29 
Classical drug therapies have generally focused upon interactions with proteins in efforts to modulate their disease-causing or disease-potentiating functions. Such therapeutic approaches have failed for cytomegalovirus infections. Effective therapy for CMV has not yet been developed despite studies on a number of antiviral agents. Interferon, transfer factor, adenine arabinoside (Ara-A), acycloguanosine (Acyclovir) and certain combinations of these drugs have been ineffective in controlling CMV infections. Based on preclinical and clinical data, foscarnet and ganciclovir show limited potential as antiviral agents. Foscarnet treatment has resulted in the resolution of CMV retinitis in five AIDS patients to date. Ganciclovir studies have shown efficacy against CMV retinitis and colitis. However, though ganciclovir seems to be well tolerated by most treated individuals, the appearance of a reversible neutropenia, the emergence of resistant strains of CMV upon long-term administration, and the lack of efficacy against CMV pneumonitis limit the long term applications of this compound. Cidofovir was approved to treat CMV in certain AIDS patients due to its undesired toxicities. The development of more effective and less toxic therapeutic compounds and methods is needed for both acute and chronic use.
Several HCMV vaccines have been developed or are in the, process of development. Vaccines based on live attenuated strains of HCMV have been described. A proposed HCMV vaccine using a recombinant vaccinia virus expressing HCMV glycoprotein B has also been described. However, vaccinia models for vaccine delivery are believed to cause local reactions. Additionally, vaccinia vaccines are considered possible causes of encephalitis.
Varicella zoster virus (VZV) is the etiologic agent that produces both varicella (chickenpox) and zoster (shingles). As with other herpes viruses, VZV causes both an acute illness and lifelong latent infection. Acute primary infection (varicella) typically occurs during childhood, where the resulting infection is relatively mild. Conversely, primary infection in adults can be more severe. Herpes zoster cutaneous eruptions are caused by reactivation of VZV present in sensory ganglia.30 Herpes zoster occurs more frequently with elderly and immunosuppressed individuals, and is eight times more likely to develop in HIV-infected individuals than in other individuals in comparable age groups.31 
Along with other immunosuppressed patients, HIV-infected patients may develop severe and in certain cases life-threatening illnesses following either primary or recurrent VZV infection. Therapy for HIV-infected patients experiencing VZV infection generally involves administering acyclovir or vidarabine (Ara-A), with hospitalization required in many instances. To inhibit VZV replication, serum levels of acyclovir are about ten times greater than those needed to inhibit Herpes Simplex Type 1 and 2.
Herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2) can establish latency following primary infection and can thus subsequently reactivate to induce recurrent disease. Upon primary infection, herpes simplex type I induces diseases including primary gingivostomatitis, encephalitis, and kerato-conjunctivitis, while herpes simplex type 2 induces primary genital herpes and neonatal herpes. Upon recurrence, herpes simplex type 1 induces diseases including recurrent oral herpes and recurrent kerato-conjunctivitis, while herpes simplex type 2 induces recurrent genital herpes.32 HSV infection in HIV-infected patients can produce widespread and occasionally life-threatening lesions.
Acyclovir, delivered either intravenously, orally, or topically, shortens clinical illness in both immunocompetent and immunosuppressed patients. Vidarabine also has been used in treating HSV. Some vaccine strategies have been investigated with a view towards preventing initial primary infection. However, protecting only against primary disease but not protecting against latency and subsequent recurrence is inadequate for those persons already initially infected. Moreover, acyclovir-resistant HSV infections recently have been observed, in many cases occurring among HIV-infected patients treated successfully with acyclovir in the past. The existence of such acyclovir-resistant infections in HIV-infected patients is troubling in view of the limited number of alternative therapeutic options available.
Respiratory Syncytial Virus (RSV) is the prime etiologic agent producing lower respiratory tract disease. RSV causes extensive yearly epidemics during which there is a marked increase in hospital admissions of patients, especially infants and young children, experiencing severe lower respiratory tract disease. Immunosuppressed patients infected with RSV are at high risk of mortality. Ribavirin is the only currently approved drug for treating RSV infections. However, this drug appears to have limited efficacy. Additionally, development of effective vaccines has proven difficult to date.
The viruses described above can act as sole causes of infection or can act to produce opportunistic infections in patients already battling immunosuppressing infections such as HIV. Acting by themselves, these viruses can present therapeutic challenges. But when acting to produce opportunistic infections in HIV-infected or other immunosuppressed patients, these viruses dramatically increase the difficulty and complexity of successful treatment.
In addition to the viruses discussed above, other viral, bacterial, fungal, and protozoal pathogens can induce opportunistic infections. Common opportunistic pathogens in addition to those described above include Mycobacterium avium complex (MAC), Pneumocystis carinii (C), and M tuberculosis. 
Present therapies for HIV-infected patients also suffering from opportunistic infection generally involve administering a plurality of antiviral compounds. In such a treatment regimen, termed combination therapy, each antiviral compound employed demonstrates best antiviral activity against a distinct viral infection. For example, a combination therapy of AZT and ganciclovir can be used for an HIV-infected patient also experiencing CMV retinitis, where AZT targets the HIV infection and ganciclovir targets the CMV infection. Thus, combination therapies can be powerful therapeutic tools. Even more powerful and desirable, however, would be a single antiviral compound that demonstrates antiviral activity against both HIV and other viruses.
While some limited success has been realized in the search for viable therapeutics for treatment of the viral infections discussed above, therapeutic agents for many viruses remain severely limited. Furthermore, there are no known safe and therapeutic treatments for HBV, influenza and HIV. In HBV, with the possible exception of the drug 3TC, the use of nucleoside-based antiviral agents leads to toxicity, probably due to cross-inhibition of cellular mitchondrial DNA. Clearly, there is a need for a new class of antiviral agents which could minimize the toxicity associated with cross-inhibition. In influenza, amantadine and rimantadine have been shown to be moderately effective against only influenza A viruses, with amantadine having excessive side effects. Recently, strains of influenza A resistant to amantadine and rimantadine have been isolated. Accordingly, there is a need for new types of therapeutic antiviral agents particularly against both influenza A and influenza B, as well as against HIV, HBV and HIV and other viruses. Furthermore, due to the loss of CD4 T lymphocytes in an HIV infected person, leading to immunodeficiency and thus increasing susceptibility to a broad range of opportunistic viral, bacterial, fungal, and protozoal pathogens, identifying anti-HIV agents having a spectrum of antiviral and antimicrobial activities is of particular interest. These agents would be not only effective against HIV infection, but also effective against or preventive of opportunistic infections in AIDS patients.
A class of coumarin compounds, either natural products isolated from several tropical plants of the genus Calophyllum3, 33-38 or synthetic analogues,39-41 have been demonstrated to be active against HIV-1. (+)-Calanolide A (1), the most active one in this class, has been selected for further pharmacological and clinical development. However, a natural source of (+)-calanolide A is limited. This limited availability fueled the desire to develop practical synthesis routes to enable further study and development to be carried out on this active and promising series of compounds. Herein, we describe improved synthetic approaches for the synthesis of important intermediates for the synthesis of antiviral calanolide compounds.
The present invention relates to synthetic approaches for preparing antiviral calanolides and key intermediates thereof. A number of total syntheses of calanolide compounds have been reported by us4446 and by others.47-51 In our previous syntheses (FIG. 1),44-46 2,2-dimethyl-5-hydroxy-6-propionyl-10-propyl-2H,8H-benzo[1,2-b:3,4bxe2x80x2]dipyran-8-one (4) was the key intermediate, which was derived from 5,7-dihydroxy-8-propionyl-4-propylcoumarin (3). In this application, we report a novel approach to the synthesis of 2,2-dimethyl-5-acyloxy-10-propyl-2H,8H-benzo[1,2-b:3,4-bxe2x80x2]dipyran-8-one (5) and 2;2-dimethyl-5-hydroxy-10-propyl-2H,8H-benzo[1,2-b:3,4-b]dipyran-8-one (6) (FIGS. 3 and 4). These compounds serve as important intermediates for the synthesis of antiviral calanolide compounds. For example, Fries rearrangement52-54 on compound 5a, or Friedel-Crafts reaction on 6, yields intermediate 4, which, in turn, can be converted to (+)-calanolide A and (xe2x88x92)-calanolide B44-46 (FIGS. 1 and 5).
Thus, in one embodiment of the invention, a method for preparing (+)-calanolide A using 6 is provided. According to the method, 6 is coupled to compound 11 to provide compound 13. Compound 13 is subsequently hydrolyzed to produce 13 (Y=OH). 25 Compound 13 (Y=OH) is then subjected to an intramolecular Friedel-Crafts reaction to provide ketone (+)-10. Ketone (+)-10 is then subjected to Luche reduction to produce (+)-calanolide A 1 (FIG. 6).
Alternatively, compound 13 may be prepared from compound 6 by treating the latter compound with 12 to produce 14. Compound 14 is then subject to a Swern oxidation to produce aldehyde 13 (Y=H). Compound 13 (Y=H) is subsequently oxidized to provide 13 (Y=OH) (FIG. 7).
In another embodiment of the invention, a general method for providing calanolide analogues is provided. According to this method, 1,3,5-triphenol is reacted with xcex2-keto 25 ester under Peckman conditions.44-46 The resulting compound 15 is then selectively acylated to provide compound 16. Compound 16 is then chromenylated to produce compound 17.44-46 Hydrolysis of 17 produced compound 18 which is then coupled with ester 19 to produce 20. Compound 20 was then cyclized under Friedel-Crafts conditions to produce 21. Reduction of 21 with a suitable reagent, e.g. sodium borohydride, results in calanolide analogue 22 (FIG. 8).4446 e 
Alternatively, compound 20 may be prepared from compound 18 by coupling the latter compound with 23 to produce 24. Compound 24 was then subject to swern oxidation to produce the aldehyde analogue 20 (Yxe2x95x90H). The aldehyde analogue was then oxidized to produce the carboxylic analogue 20 (Yxe2x95x90OH) (FIG. 9).
These and other embodiments of the invention will become apparent in light of the detailed description below.
FIG. 1 is a schematic illustrating the conversion of compounds 5(a) and 6 to 4, a key intermediate for the preparation of calanolides such as (+)-calanolide A.
FIG. 2 is a retrosynthetic schematic illustrating the conversion of intermediates 2 to 7 to 5a.
FIG. 3 illustrates selective acylation reactions of coumarin 2 to form compounds 7a-c and 8a-c.
FIG. 4 illustrates the chromenylation of coumarin compounds 7a-c to 5a-b and the basic hydrolysis of acylated chromeno-coumarin 5a-b to compound 6.
FIG. 5 illustrates the conversion of key intermediates 5a and 6 to (xc2x1)-calanolide A.
FIGS. 6 and 7 further illustrate preparation of (+)-calanolide A from 6.
FIGS. 8 and 9 illustrate preparation of calanolide analogues.
The present invention relates to methods for preparing (+)-calanolide A and calanolide analogues. In one embodiment of the invention, a novel approach to the synthesis of 2,2-dimethyl-5-acyloxy-10-propyl-2H,8H-benzo[1,2-b:3,4-bxe2x80x2]dipyran-8-one (5) and 2,2-dimethyl-5-hydroxy-10-propyl-2H,8H-benzo[1,2-b:3,4-bxe2x80x2]dipyran-8-one (6) is provided. These compounds serve as important intermediates for the synthesis of antiviral calanolide compounds. For example, Fries rearrangement on compound 5a, or Friedel-Crafts reaction on 6, yields intermediate 4, which, in turn, can be converted to (+)-calanolide A and (xe2x88x92)-calanolide B (FIGS. 1 and 5).
The strategy for synthesis of compound 5a is based on the fact that under the modified Friedel-Crafts reaction conditions 5-hydroxy-7-propionyloxy4-propylcoumarin (7a) was selectively formed from 2 in 10% yield.45 Chromenylation44,45 on 7a is expected to deliver compound 5a (FIG. 2). Due to the low yields from the Friedel-Crafts acylation reaction, a more practical procedure needed to be developed for the synthesis of 7a from 2. A variety of reaction conditions were investigated, which is summarized in Table I in Example 1. Accordingly, improved acylation conditions were discovered which provided surprisingly increased production yields of 7a.
In conducting this reaction, a solution of suitable acylating agent, e.g., acyl chloride or anhydride, in a suitable solvent, e.g.,THF, was added in a dropwise manner to a vigorously stirred solution of 5,7-dihydroxy4-propylcoumarin 2 , a Lewis acid catalyst or a catalytic amount of a base, and an organic solvent cooled in an ice bath. Dropwise addition of the acylating agent is conducted such that the temperature of the reaction mixture is maintained at a temperature ranging between 0xc2x0 C. and about 30xc2x0 C.
In making compounds of the invention, the amount of acylating agent used generally ranges between about 0.5 and about 6 moles, preferably ranging between about 1 and about 2 moles, per mole of 2.
Non-limiting examples of Lewis acid catalysts useful in the acylation reaction include AlCl3, BF3, SnCl4, ZnCl2, POCl3 and TiCl4. A preferred Lewis acid catalyst is AlCl3. The amount of Lewis acid catalyst relative to 5,7-dihydroxy-4-propylcoumarin, 2, ranges between about 0.5 and about 12 moles, preferably ranging between about 2 and about 5 moles, per mole of 5,7-dihydroxy-4-propylcoumarin, 2.
Non-limiting examples of a base useful in the acylation reaction include pyridine and 4-dimethylaminopyridine(DMAP). Catalytic amounts (0.1 eq) of the base may be used in combination with a suitable reaction solvent. Alternatively, the base may be used as the reaction solvent, however, complex product mixtures may results (Example 1).
Non-limiting examples of organic solvent for use in the acylation reaction include THF, dichloroethane, pyridine, and mixtures thereof.
Upon completion of the addition of acylating agent, the vigorously stirred reaction mixture is maintained at a temperature ranging between about 0xc2x0 C. and about 30xc2x0 C., preferably about room temperature (25xc2x0 C.) until the reaction reaches completion as monitored by conventional means such as TLC analysis. The reaction mixture is then poured onto ice and extracted several times with a suitable solvent such as ethyl acetate, chloroform, methylene chloride, tetrahydrofuran, or a mixture of chloroform/methanol. A preferred solvent for this extraction is ethyl acetate. The extracts are then dried over a suitable drying agent, e.g., sodium sulfate, and the product may be purified by conventional means such as silica gel column chromatography.
Chromenylation of 7a was initially attempted employing 4,4-dimethyoxy-2-methylbutan-2-ol according to the literature method,44,45 and only ca. 5% of 5a was detected by 1H NMR. However, when 3-chloro-3-methyl-1-butyne was used,47,55 5a was obtained in 27% isolated yield (FIGS. 4 and 5). The same procedure on 7b afforded 5b in 73% yield. In contrast, no 5c could be detected when 7c was reacted with 3-chloro-3-methyl-1-butyne under the same conditions. Instead, a tripyranone derivative 956 was formed. The structure assignment of 9 was based on 1H NMR and MS (FIG. 4). This indicated that the TBMDS-protecting group was lost during the course of chromenylation.
Hydrolysis of 5a to produce 6 under basic conditions proceeded smoothly. For example, conversion of 5a to 6 was uneventful with sodium bicarbonate in aq. MeOH in 44% yield (FIGS. 4 and 5). This represents a substantial yield improvement over previous methods for preparing 6. For instance, prior reported direct chromenylation of 2 with 4,4-dimethyoxy-2-methylbutan-2-ol furnished a mixture of product, with 6 being isolated in less than 10% yield55. Fries rearrangement on 5a or Friedel-Crafts reaction on 6 led to intermediate 4 which can then be converted to (+)-calanolide A and (xe2x88x92)-calanolide B (FIG. 1) using previously reported procedures.44-46 
FIG. 5 illustrates preparation of (xc2x1)-calanolide A from 5a or 6. Thus, treatment of 2 using the inventive acylation method resulted in 7a. Chromenylation of 7a with 3-chloro-3-methyl-1-butyne resulted in 5a which can be converted directly to 4 via Fries rearrangement in the presence of AlCl3. Alternatively, 4 may be obtained by the hydrolysis of 5a to yield 6, followed by a Friedel-Crafts reaction as discussed above.
The conversion of 5a to 4 under Fries rearrangement conditions is straightforward. A mixture of compound 5a with anhydrous aluminium chloride in an amount ranging from about 0.1 to about 20 moles, preferably about 10 moles, per mole of compound 5a is heated for a time period ranging between about 0.5 to about 6 hours, preferably about 2 hours at a temperature ranging between about 40xc2x0 C. to about 250xc2x0 C., preferably about 160xc2x0 C. The mixture is then cooled to r.t. and treated with ice and hydrochloric acid to precipitate out the product. The precipitated product is taken into a suitable solvent, preferably ethyl acetate, and the aqueous solution is extracted with the same solvent. The extracts are combined and dried over any suitable drying agent, e.g., Na2SO4 and concentrated in vacuo. The crude product thus obtained may then be purified by any suitable means such as silica gel column chromatography using any suitable solvent or solvent mixtures, e.g., 25% ethyl acetate in hexane.
Compound 4 can be converted to 12-oxocalanolide 11 using a variety of reagents including chlorotitanium-mediated aldol reaction with acetaldehyde followed by the Mitsunobu reaction, and treatment with paraaldehyde or CH3CH(OEt)2 as described in U.S. Pat. Nos. 5,489,697; 5,869,324; 5,874,591; 5,840,921; 5,847,164; 5,892,060; 5,872,264; 5,981,770; 5,977,385; 6,043,271; and co-pending application 09/173,143, filed Oct. 15, 1998 which are incorporated by reference in their entirety. Luche reduction of 11 with NaBH4 and CeCl3(H2O))7 yields (xc2x1)-calanolide A as described in U.S. Pat. Nos. 5,489,697; 5,869,324; 5,874,591; 5,840,921; 5,847,164; 5,892,060; 5,872,264; 5,981,770; 5,977,385; 6,043,271; and co-pending application 09/173,143, filed Oct. 15, 1998.
FIG. 6 and FIG. 7 further illustrate preparation of (+)-calanolide A from compound 6. According to FIG. 6, a method for preparing (+)-calanolide A using 6 is provided. The introduction of the chiral side chains at the desired 7-position of 6 can be achieved using a variety of readily available chiral compounds 1157-60 and 12. The latter compound, 12 (Z=H), is resulted from reduction of 11 (X=OH, Y=OMe) with LiAlH4. The primary OH group in 12 (Z=H) is then selectively protected such that Z is, for example, t-butyldimethylsilyl (TBDMS), tetrahydropyran (THP), p-toluenesulfonyl (Ts) or COR10 wherein R10 represents C1-6 alkyl, aryl-C1-6 alkyl, mono- or poly-fluorinated C1-6 alkyl, aryl or heterocycle. According to the method, 6 is coupled to compound 11 (X=OH) under Mitsunobu conditions (PPh3, diethyl azodicarboxylate) to provide compound 13. Compound 13 is subsequently hydrolyzed to produce 13 (Y=OH). Compound 13 (Y=OH) is then subjected to an intramolecular Friedel-Crafts reaction to provide ketone (+)-10. Ketone (+)-10 is then subjected to Luche reduction to produce (+)-calanolide A 1.
The reaction of 6 with 11 (X=TsO) under nucleophilic substitution conditions also generates compound 13. Hydrolysis (NaOH, LiCl) of 13 (Y=OMe), or removal of the chiral auxiliary (LiOH or LiOOH) from 13 (Y=oxazolidinone), followed by intramolecular Friedel-Crafts cyclization, yields the chromanone (+)-10. Luche reduction of (+)-10 affords (+)-calanolide A. It should be noting that a substantial elimination from 11 (X=OH or TsO) occurred under both Mitsunobu and the nucleophilic substitution conditions, resulting in a requirement for excessive amount of the chiral moiety and a reduction in yield of 13.
In order to avoid the xcex2-elimination from 11, a selectively protected chiral diol compound 12 is devised. Thus, Mitsunobu reaction (PPh3, diethyl azodicarboxylate) of 6 with 12 (Z=TBDMS) leads to the formation of 14 (Z=H), followed by removal of TBDMS protecting group (FIG. 7). No xcex2-elimination from 12 was observed in this process. Swern oxidation of 14 (Z=H) furnishes aldehyde derivative 13 (Y=H), which is further oxidized using NaClO2 to form the carboxylic acid, 13 (Y=OH). As described in FIG. 6, intramolecular Friedel-Crafts cyclization on 13 (Y=OH) followed by Luche reduction yields (+)-calnolide A.
The synthetic sequence for (+)-calanolide A is extended to the synthesis of calanolide analogues (FIGS. 8 and 9). Thus, Pechmann reaction of phloroglucinol with various xcex2-ketoesters yields substituted 5,7-dihydroxycoumarin 15. Selectively protecting the 7-hydroxy group leads to the formation of 16. Chromenylation of 16 can be achieved by reacting with xcex2-hydroxyaldehyde dimethylacetal, providing chromenocoumarin 17, which is deprotected to furnish the free hydroxy group in 18. Mitsunobu reaction of 18 with 19 (X=OH), or nucleophilic substitution with 19 (X=TsO), followed by the hydrolysis, results in 20 (Y=OH). Intramolecular Friedel-Crafts cyclization on 20 (Y=OH) gives chromanone 21, which is reduced by NaBH4 to form the final calanolide analogues 22.
According to FIG. 8, 1,3,5-trihydroxybenzene was reacted with xcex2-keto ester 25 under Pechmann conditions (See U.S. Pat. Nos. 5,489,697; 5,869,324; 5,874,591; 5,840921; 5,847,164; 5,892,060; 5,872,264; 5,981,770; 5,977,385; 6,043,271; and co-pending application 09/173,143, filed Oct. 15, 1998, incorporated by reference in its entirety) to produce compound 15. The amount of xcex2-keto ester 25 to 1,3,5-trihydroxybenzene generally ranges between about 1 to about 3, preferably about 1 per mole of 1,3,5-trihydroxybenzene. xcex2-ketoester 25 is represented by the structure: 
wherein R1 is H, halogen, hydroxyl, amino, C1-6 alkyl, aryl-C1-6 alkyl, mono- or poly-fluorinated C1-6 alkyl, hydroxy-C1-6 alkyl, C1-6 alkoxy, amino-C1-8 alkyl, C1-6 alkylamino, di(C1-6 alkyl)amino, C1-8 alkylamino-C1-8 alkyl, di(C1-6 alkyl) amino-C1-8 alkyl, cyclohexyl, aryl, or heterocycle, wherein the aryl or the heterocycle may each be unsubstituted or substituted with one or more of the following: C1-6 alkyl, C1-6 alkoxy, hydroxy-C1-4 alkyl, hydroxyl, amino, C1-6 alkylamino, di(C1-6 alkyl)amino, amino-C1-8 alkyl, C1-8 alkylamino-C1-8 alkyl, di(C1-6 alkyl-amino-C1-8 alkyl, nitro, azido or halogen; and R2 is H, halogen, hydroxyl, C1-6 alkyl, aryl-C1-6 alkyl, mono- or poly-fluorinated C1-6 alkyl, aryl or heterocycle. Compound 15 is represented by the structure: 
wherein R1 and R2 are as described above.
Thereafter, compound 15 is acylated with an acylating agent (RCO)2O under conventional acylation conditions to produce 16 wherein R represents C1-6 alkyl, aryl-C1-6 alkyl, mono- or poly-fluorinated C1-6 alkyl, aryl or heterocycle. The amount of acylating agent to compound 15 generally ranges between about 0.5 to about 6, preferably about 1 per mole of 15. Compound 16 is represented by the structure: 
wherein R, R1, and R2 are as described above.
Compound 17 is produced by chromenylation of 16 with substituted xcex2-hydroxyaldehyde dimethylacetal 26 under the reaction conditions described in U.S. Pat. Nos. 5,489,697; 5,869,324; 5,874,591; 5,840,921; 5,847,164; 5,892,060; 5,872,264; 5,981,770; 5,977,385; 6,043,271; and co-pending application 09/173,143, filed Oct. 15, 1998, incorporated by reference in their entirety. Representative examples of substituted xcex2-hydroxyaldehyde dimethylacetal 26 comprise: 
wherein R3 and R4 are independently selected from the group consisting of H, C1-6 alkyl, aryl-C1-6 alkyl, mono- or poly-fluorinated C1-6 alkyl, aryl or heterocycle; R3 and R4 can be taken together to form a 5-7 membered saturated cycle ring or heterocycle ring; and R5 is H, halogen, methyl, ethyl. Compound 17 is represented by the structure: 
wherein R, R1, R2, R3, R4, and R5 are as described above.
Thereafter, compound 17 is hydrolyzed to produce compound 18 under the basic hydrolysis conditions described above. Compound 18 is then coupled to 19 under Mitsunobu conditions to produce compound 20. Compound 19 is represented by the structure: 
wherein R6, R7, R8 and R9 are independently selected from the group consisting of H, halogen, hydroxy, amino, mono- or dialkylamino-C1-6 alkyl, aryl-C1-6 alkyl, mono- or poly-fluorinated C1-6 alkyl, hydroxy-C1-6 alkyl, amino-C1-6 alkyl, aryl or heterocycle; or R6 and R7 together form a 5 to 7 membered cycle ring or heterocycle ring; or R8 and R9 together form a 5 to 7 membered cycle ring or heterocycle ring.
Compound 20 is represented by the structure: 
wherein R1-9 are as described above and Y represents hydrogen, OH, or OMe.
Friedel-Crafts cyclization of 20 under conditions described above provides compound 21 which structure is represented below. 
wherein R1-9 are as described above.
Reduction of compound 21 using any suitable reagent, e.g., sodium borohydride, results in calanolide analogue 22 whose structure is represented below. 
wherein R1-9 are as described above.
Except as expressly defined otherwise, the following definition of terms is employed throughout this specification.
The terms xe2x80x9calkylxe2x80x9d, xe2x80x9clower alkylxe2x80x9d or xe2x80x9cC1-6 alkylxe2x80x9d mean a straight or branched hydrocarbon having from 1 to 6 carbon atoms and includes, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, and the like. The alkyl group can also be substituted with one or more of the substituents listed below for aryl.
By xe2x80x9calkoxyxe2x80x9d, xe2x80x9clower alkoxyxe2x80x9d or xe2x80x9cC1-6 alkoxyxe2x80x9d in the present invention is meant straight or branched chain alkoxy groups having 1-6 carbon atoms, such as, for example, methoxy, ethoxy, propoxy, isopropoxy, n-butoxy, sec-butoxy, tert-butoxy, pentoxy, 2-pentyl, isopentoxy, neopentoxy, hexoxy, 2-hexoxy, 3-hexoxy, and 3-methylpentoxy.
The term xe2x80x9chalogenxe2x80x9d includes chlorine, fluorine, bromine, and iodine, and their monovalent radicals.
The term xe2x80x9carylxe2x80x9d means an aromatic carbocyclic group having a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or multiple condensed rings in which at least one is aromatic (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl), unsubstituted or substituted by 1 to 3 substituents selected from alkyl, 0-alkyl and S-alkyl, OH, SH, xe2x80x94CN, halogen, 1,3-dioxolanyl, CF3, NO2, NH2, NHCH3, N(CH3)2, NHCO-alkyl, xe2x80x94(CH2)mCO2H, xe2x80x94(CH2)mCO2-alkyl, xe2x80x94(CH2)mSO3H, xe2x80x94NH alkyl, xe2x80x94N(alkyl)2, xe2x80x94CH2)mPO3H2, xe2x80x94(CH2)mPO3(alkyl)2, xe2x80x94(CH2)mSO2NH2, and xe2x80x94(CH2)mSO2NH-alkyl wherein alkyl is defined as above and m is 0, 1, 2, or 3.
The term xe2x80x9ccyclic ringxe2x80x9d as referred to herein means a monocyclic or polycyclic moiety. By xe2x80x9cpolycyclicxe2x80x9d is meant two or more rings that share two or more carbon atoms. A xe2x80x9ccarbocyclic groupxe2x80x9d which contains hetero atoms as one or more of its members can be referred to as a xe2x80x9cheterocyclexe2x80x9d or a xe2x80x9cheterocyclic ringxe2x80x9d. Such a xe2x80x9cheterocyclexe2x80x9d can likewise be xe2x80x9cmonocyclicxe2x80x9d or xe2x80x9cpolycylcicxe2x80x9d. A cyclic ring and a heterocyclic ring can be saturated, can contain one or more double bonds or can be aromatic. Each ring can be unsubstituted or substituted by 1 to 3 substituents selected from the group as described above for aryl.
The following examples are illustrative and do not serve to limit the scope of the invention as claimed. All references, patents and patent applications cited herein are hereby incorporated by reference in their entirety.
General: Melting points were uncorrected. All commercial reagents and solvents were used without further purification. The 1H NMR (300 MHz) and 13C NMR (75 MHz) were run in indicated deuterated solvent and chemical shifts are reported in ppm with tetramethylsilane as the internal standard.